1. Department of Histology & Embryology; College of Basic Medical Sciences; Jilin University; Changchun 130021 People's Republic of China
2. Department of Anatomy; Changchun Medical College; Changchun 130021 People's Republic of China
3. Department of Pathology; Affiliated Hospital of Jilin Medical College; Jilin 132013 People's Republic of China
4. Department of Histology & Embryology; Guangdong Medical College; Dongguan 523808 People's Republic of China